1. Field of the Invention
The present invention relates to a polyvinyl alcohol polymer film which is useful as a production material of a polarization film having few color irregularities or mottles and is not wrinkled easily in stretching, and method of producing the same and a polarization film.
2. Description of the Prior Art
A polarizer having functions of transmission and shielding of light is a fundamental constituent element of a liquid crystal display (LCD), together with a liquid crystal having a function of switching light. Applied fields of such an LCD have expanded from small apparatuses such as electronic calculators, wristwatches and the like in the initial period of its development, to, in the recent years, apparatuses in a wide range such as lap top personal computers, word processors, liquid crystal color projectors, navigation systems for automobiles, liquid crystal televisions, personal phones, indoor and outdoor measuring apparatuses and the like, and there is a need for polarizers manifesting fewer color irregularities and having larger area than conventional products.
For obtaining a polarizer having general constitution, a polyvinyl alcohol polymer film (hereinafter, polyvinyl alcohol polymer is abbreviated as xe2x80x9cPVAxe2x80x9d, and a polyvinyl alcohol polymer film is abbreviated as a xe2x80x9cPVA filmxe2x80x9d, in some cases) is monoaxially stretched before dyeing, or dyed before monoaxial stretching, and subjected to fixing treatment with a boron compound (in some cases, dyeing and fixing treatment are conducted simultaneously), then, a protection film such as a triacetic cellulose (TAC) film, cellulose acetate butyrate (CAB) and the like is laminated on the polarization film.
However, even in the case of a PVA film being apparently uniform, a polarization film obtained from such PVA film may include color irregularities which are not observed easily from outside. These color irregularities are recognized easily only in the final product (polarizer) obtained by laminating a protective film and the like. When color irregularities are manifested in this final product, also indirect materials such as a protective film and the like which are good items are discarded as defective items, leading to large loss in cost. Conventionally, for reducing color irregularities, decrease in thickness irregularity and birefringence irregularity has been investigated, as described, for example, in Japanese Laid-open Patent Publication No. 6-138319. By decreasing thickness irregularity and birefringence irregularity, it has made possible to decrease color irregularities to a certain extent and to satisfy the level required at that time, however, it has been found that it is difficult to reduce color irregularities at the level problematical in the final product (polarizer) having recent increased performance. Further, it has been found that even if thickness irregularity and birefringence irregularity are evaluated as irregularities of the whole film surface, they dot not correctly correspond to actual color irregularities perceived by human eyes.
Further, for obtaining a polarization film having larger area along with increased size of a liquid crystal display screen, it is necessary to obtain a wide PVA film. However, in conventional wide PVA films, curl easily occurs to cause wrinkle on both end parts, leading to poor yield along cross direction, in stretching, consequently, a polarization film having large area is not obtained easily. Particularly, wrinkle is easily produced in films having width of 2 m or more.
Therefore, an object of the present invention is to provide a PVA film useful as a production raw material of a wide polarization film which manifests fewer color irregularities, does not produce wrinkle easily in stretching, and can realize increase in size of a liquid crystal display screen.
For attaining the above-mentioned objects, in the PVA film of the present invention, difference in retardation between two points separated by 1 cm along the TD direction of the film is 5 nm or less.
Here, the retardation is represented by xe2x80x9cbirefringencexc3x97film thicknessxe2x80x9d of a PVA film. This birefringence is determined by a degree of molecular orientation of the above-mentioned film imparted in a film production process and the like. Further, the machine direction (MD direction) of a PVA film is the film longitudinal direction, and the transverse direction (TD direction) to this machine direction is the film width direction.
According to the present invention, by setting difference in retardation between two points separated by 1 cm along the TD direction of a PVA film as 5 nm or less, a wide good item (polarization film) which manifests fewer color irregularities and does not easily produce wrinkle in stretching due to unknown reason can be produced in high yield, enabling increase in size of a liquid crystal display screen.
In the production method of the present invention, a surface for drying a PVA film has a length from 3 m to 20 m, and the water content of the PVA film when peeled after passing through this drying surface is 50% by weight or less.
According to the present invention, a PVA film useful as a production raw material of the intended polarization film can be obtained infallibly.